Fire alarming system

ABSTRACT

An ionization smoke detector having means for briefly reducing the voltage across the series connected ionization chambers when the detector is actuated by either smoke or the failure of a component part. The detector therefor, when actuated by smoke will produce a periodic signal as the voltage is reduced and returns again to the normal voltage. However, if the detector is defective a continuous signal is produced. Through the use of a discriminator connected to the detector the periodic signal will produce an alarm while the continuous signal will provide an indication of a faulty detector.

United States Patent Sasaki 1 Nov. 7, 1972 54] FIRE ALARMING SYSTEM3,564,524 r 2 /1971 Walthard e't-aL340/237 s UX [72] Inventor; Kojisasaki, Tokyo, Japan 3,588,842 6/1971 Scheidweiler....340/237 S UX3,603,949 9/1971 Walthard ..340/237 S UX Asslsnw (30min! mild, 3,657,7134/1972 Kojusasaki et al. .340/237 s x Japan 3,665,461 5/1972 Gniigi eta1 ..340/256 X [22] Filed: July 28, 1971 Pr E I J h w C M u i imaryxammero n a we [21] Appl' 166356 Assistant Examiner-Daniel MyerAttorney-Eugene E. Geoffrey [30] Foreign Application Priority Data Aug.10, 1970 Japan ..45/69275 [571 a ABSTRACT 10, 1970 Japan An ionizationsmoke detector having means for briefly Aug. 20, 1970 Japan reducing thevgltage across the series connected 31, 1970 J p --45/75628 ionizationchambers when the detector is actuated by either smoke or the failure ofa component part. The [52] US. Cl ..340/237 S, 340/409 detectortherefor, h actuated by Smoke will [51] Int. Cl. ..G08b 17/10, G08b29/00 produce a periodic signal as the voltage is reduced and Field ofSearch-340m 256; returns again to the normal voltage. However, if the250/83'6 44 detector is defective a continuous signal is produced.Through the use of a discriminator connected to the [56] ReferencesC'ted detector the periodic signal will produce an alarm UNrTED STATESPATENTS while the continuous signal will provide an indication of afaulty detector. 2,646,556 7/1953 j Allen ..340/409UX 2,782,404 2/ 1957Bergman ..340/409 X 9 Claims, 10 Drawing Figures 3,160,866 12/1964 Meili..340/237 S UX 3,530,450 9/1970 Walthard et al..340/2 37 S UX VOLTAGESOURCE VOLTAGE CHANGER ION I ZATION snow; DETECTOR DISCRIHINATOINFORM/N5 DEV/CE PATENTEDuuv 1 m2- 3,702,468

sum 1 or 3 "f"'"""""" T g-""7 l6 /J/7 l g i 1: i 5 4 2% i 4 E L H IBRIOR ART VOLTAGE VOLTAGE N [5 SOURCE VOLTAGE CHANGER IONIZATIONINFORM/MG SMOKE DETECTOR DlSCRlH/NATO DEVICE FIRE ALARMING' SYSTEM Thisinvention relates to a fire alarm system, especially a fire alarm systemof a type utilizing ionization smoke detectors and more particularly analarm system capable of distinguishingthe erroneous actuation of the.detector from its true actuation and indicating the erroneous actuationseparately from the true actuation.

Prior ionization type fire alarm systems include a plurality of localunits consisting of ionization smoke detectors connected in-parallelbetween a pair of conductors extending from a central unit including apower supply and a relay connected in series between the pair ofconductors and an alarm device responding to the relay. The ionizationsmoke detector constituting each local unit includes apair of ionizationchambers each .having a pair of electrodes and a radioactive source forionizing the atmosphere in the chamber, one chamber being closed to theexternalair and referred as closed ionization chamber while the otherbeing open to the external air and referred as open ionization chamber,the both chambers being connected in series between the pair ofconductors. A field effect transistor (hereinafter referred to as FET)having a gate electrode is connected to the connection point of bothionization chambers and a ,drain-to-source conduction path is connectedin series'with a load resistor between the pair of conductors. A siliconcontrolled rectifier (hereinafter referred to as SCR) having a controlelectrode is connected through aZener diode to the source electrode ofthe FET and the conductor path is connected between the pair ofconductors.

When smoke enters the open ionizationchamber, the ionization current inthe open ionization chamber is reduced to raise the potential at theconnection point of both ionization chambers and accordingly at the gateelectrode of the FET. This results in a rise of the source potential ofthe PET and when it exceeds thezener voltage of the Zener diode it-isapplied to-the control electrode of the SCR to drive it intoconductionto short-circuit the pair of conductors. Thus, therelayis energized toactuate the alarm device.

In the suchprior fire alarm systems'an alarm is also sounded when theionization smoke detectoroperates erroneously due to breakdown oftheFET,'insufficient insulation and the like, even if smoke does notenter the open ionization chamber. This is a very troublesomephenomenon.

Accordingly, the principal object of this invention resides in theprovision of a fire alarm system which can distinguish erroneousactuation of the ionization smoke detector from its true actuation andindicate it separately from the true actuation.

This object is attained in accordance with this invention by providingmeans for reducing the applied voltage beiow the enabling voltage of theionization smoke detector in response to the actuation of the detectorand means for detesting the mode of current change in the conductors.

The above and other features of this invention will be more clearlyunderstood from the following description with reference to theaccompanying drawings.

In the drawings FIG. 1 is a schematic circuit diagram representing atypical example of the ionization type fire alarm system according tothe prior art;

FIG. 2 is a current-to-voltage characteristic diagram of the ionizationchambers to aid in the explanation of the invention;

FIG. 3'is a block diagram representing a fire alarm system according tothis invention;

FIG. "4 is a schematic circuit diagram of one embodiment of the systemaccording to this invention;

Flg. 5 is a schematic circuit-diagram of another embodiment of thesystem according to this invention;

FIG. 6 is a schematic circuit diagram of a further embodiment of thesystem of this invention;

FIG. 7 is a schematic circuit diagram of an embodiment of the means fordiscriminating the current changing mode according to this invention;and

FIGS. 8-1 through 8-3 are charts illustrating the modes of actuation ofthe various relays in the circuit of 1 FIG. 7.

tom 3 and 4, an FET 6 having a gate electrode connected'tothe connectionpoint 5 of both ionization Throughout the drawings, like referencenumerals are'used to corresponding components.

Referring first to FIG. 1 illustrating a typical example of the priorionization type fire alarm'system, a local unit 1 and a central unit 2are connected by a pair of conductors 3 and 4. Although a plurality oflocal units are connected in parallel between the conductors 3 and 4 inthe practical system, only one unit is shown for the purposes ofsimplicity. The local unit 1 consists of a conventional ionization smokedetector including a closed ionization chamber'l0 and an open ionizationchamber 20 connected in series between the conduc chambersand adrain-source conduction path con nected through a load resistor 7betweenthe conductom 3 and 4 and an SCR 8 having a control electrodeconnected-through a Zener diode 9 to the source electrode of the FET 6and a conduction path connected also between theconductors 3 and'4. Thecontrol electrode of the'SCRS is also connected through a leakagechamber 20 similarlyincludes a pair of electrodes 21 and 22 and aradioactive source 23. The closed ionization chamber 10 itself may bereplaced by a fixed resistorthaving a comparable impedance. The centralunit 2 includes apower supply 15 and an electromagnetic relay 16connected in series between both conductors 3 and '4 and an alarm'devicecomprising a relay contact 17, a power supply 18, a speaker 24 and anindicating lamp 25.

The operation of the circuit of FIG. 1 will now be described withreference to FIG. 2. In FIG. 2 the potential at the connection point 5of both ionization chambers l0 and 20 in FIG. 1 is shown on the abscissaand the ionization current flowing through each chamber is shown on theordinate. Curve A shows the load curve of the closed ionization chamber10 representing the ionization current in the closed ionization chamber10 with respect to thevoltage at the connection point 5 with a normalapplied voltage, and Curve B shows the characteristic of the openionization chamber 20 representing the ionization current in the openionization chamber with respect to the voltage at the same point 5 undera normal operating condition and in the absence of smoke in the openionization chamber. As

the ionization currents in both of the ionization chambers are equal inthe circuit of FIG. 1, it is clear that the voltage at the point isstabilized at voltage E2rwhich corresponds to the intersection of bothcurves A and B when smoke does not exist in the open ionization chamber20. When smoke enters the open ionization chamber 20, however, thecharacteristic of theopen ionization chamber is changed to Curve B, asan example. Therefore, the voltage at point 5, that is, the gate voltageof the FET 6, is raised to E1 from E2 as shown in FIG. .2. This resultsin a corresponding rise in the source voltage of the FET 6 and, when itexceeds the Zener voltage of the Zener diode 9, it is applied to thecontrol electrode of the SCR 8 to drive it into conduction. Thus, theconductors 3 and 4 are short-circuited through the SCR 8 and the relay16in the central unit 2 is energized to close the contact 17, therebyproducing an alarm indicating the presence of tire.

Once the SCR 8 conducts it is never restored to its cut-offconditionunless the power supply is disconnected therefrom. This is alsotrue in the case of an erroneous actuation of the detector due to adamaged FET 6, broken insulation or the like and in the case ofmomentary actuation due to electrical noise, a rapid flow of air or thelike, and such erroneous actuation cannot be distinguished from a normaland true actua tion.

The block diagram of FIG. 3 represents a basic scheme of the fire alarmsystem of this invention. As shown, a voltage changer 26 is insertedbetween a voltage source 15 and the ionization smoke detector 27. Thevoltage changer 26 has the function of reducing the applied voltage to aspecific low value in response toactuation of the ionization smokedetector 27and restoring the reduced voltage to the original value inresponse to actuation of the ionization smoke detector 27 and restoringthe reduced voltage to the original value in response to deactuation ofthe detector. The ionization smoke detector 27 according to thisinvention is constructed so that it cannot be actuated at the abovementioned reduced applied voltage even when smoke enters the openionization chamber. In the normal operation of the detector when smokeenters the open ionization chamber, the detector is actuated by incomingsmoke and is soon deactuated by the reduced applied voltage produced bythe voltage changer 26 in response to the actuation of the detector. Theapplied voltage is then restored to the original high value by thevoltage changer 26 in response to the deactuation of the detector. Thus,the ionization smoke detector 27 of this invention repeats actuation anddeactuation alternately at a specific frequency and energizes the relay16 periodically.

The value of the reduced applied voltage is also selected to maintainthe relay 16 in energized condition when the ionization smoke detector27 is erroneously 4 A discriminator 28in FIG. 3 discriminates or detectsthe difference between the mode of actuation of the 'ionization smokedetector 27 in normal and abnormal operations and produces outputsignals corresponding 'to the respective modes of actuation. Aninforming or indicating device 29 in FIG. 3 receives the output signalsfrom the discriminator 28 and informs or indicates'a condition such aspresence of tire or damage to the detector. 7

- FIGS. 4, 5 and 6 represent three embodiments of a tire alarm systemaccording to this invention, which show practical arrangements of thevoltage changer 26 and the ionization-smoke detector 27 of FIG. 3. Thesystems of FIGS. 1, 2 and 3 include, respectively, a local unit 1 and acentral unit 2 connected by a pair of conductors 3 and 4 as shown inFIG. 1. In practice, a plurality of local units are connected inparallel between the conductors 3 and 4, but only one unit is shown ineach drawing for purposes of simplification.

In FIG. 4 the local unit 1 consists of an ionization smoke detectorwhich is almost similar to that of FIG. 1, but the SCR 8 in FIG. 1 issubstituted by a bipolar transistor 31. The transistor 31 has a baseelectrode connected to the Zener diode 9 and an emitter-to-collectorpath connected through another Zener diode 32 'between the bothconductors 3 and 4. The transistor 31 and the Zener diode 32 cooperateas described hereinunder to serve the function of the voltage changer 26of FIG. 3. The Zener diode 32 has an indicating lamp 33 connected inparallel therewith for indicating conduction of the Zener diode 32.

Whensmoke enters the open ionization chamber 20 of the ionization smokedetector of FIG. 4, the operation as described in conjunction with theprior detector of FIG. 1 is produced and a control voltage is producedat the base electrode of the transistor 31 to drive it into conduction.Due to the Zener diode 32, however, the conductors 3 and 4 are notshort-circuited directly and a voltage corresponding to the Zenervoltage of the Zener diode 32 remains as the reduced applied voltage asdescribed hereinbefore. If a reduced voltage is applied to the seriesconnection of both ionization chambers 10 and 20, the load curve A ofthe closed ionization chamber 10 is changed as shown by a dashed curve Ain FIG. 2 and the gate voltage of the FET 6 is correspondingly reducedto E1 from E1 even when smoke exists in the open ionization chamber 20.Therefore, if the Zenervoltage of the Zener diode 32 is selected so thatthe gate voltage El produces the source voltage of the FET 6 which doesnot exceed the Zener voltage of the Zener diode 9, the transistor 31 iscut off and the original applied voltage is restored between bothconductors 3 and 4. Thus, the transistor 31 is alternately switchedbetween conductive and non-conductive conditions and the relay 16included in the central unit 2 periodically energized as schematicallyrepresented by a waveform (16) of FIG. 8-1.

The local unit 1 of FIG. 5 is the same as that of FIG. 4 except that afixed resistor 34 is substituted for the Zener diode 32. The centralunit 2 however includes another power supply 35 which can be substitutedfor the power supply 15 by a change-over switch 16-1 actuated by therelay 16. The voltage of the power supply 35 is lower than that of thepower supply 15 and the value is selected to provide the above mentionedreduced voltage condition.

When smoke enters the open ionization chamber and the transistor 31 issimilarly driven into conduction, the relay 16 is energized to actuatethe switch 16-1 to transfer the systemfrom power supply 15 to powersupply 35 so that the applied voltage between the conductors 3 and 4 isreduced and the transistor 31 is restored to its cut-off condition inthe same manner as in the case of FIG. 4. This results in a'similarperiodic energization of the relay 16.

The system of FIG. 6 is the same as that of FIG. 5 except that theauxiliary power supply 35 and the changeover switch 16-1 are removed. Inthis system the impedance of the relay 16 is previously selected so thatthe reduced voltage meeting the above mentioned condition is producedbetween both conductors 3 and 4 when it is energized. This serves thesame function of changing applied voltage as the circuit 4 in FIGS. 4and 5 and the relay 16 is similarly energized periodically when smokeenters the open ionization chamber 20.

In the case of erroneous actuation of the detector due to a damaged FET6 or broken insulation, the detector is not restored to its originalcondition even if the applied voltage is reduced as discussed above,since the reduced voltage is previously selected so that it willmaintain the detector in the actuated condition. Thus, the relay 16 isenergized continuously as schematically shown by a waveform (16) in FIG;8-2. In the case of an erroneous actuation due to the electrical noiseor momentary flow of air, the detector is momentarily actuated and issoon restored and the relay 16 is energized as shown by the waveform(16) in FIG. 8-3.

FIG. 7 illustrates an'embodiment of the discriminator 28 of FIG. 3. Itincludes a series connection of a normally opened switch 16-2. Aresistor 44 and a capactior 45 which form a time constant circuit. Thiscircuit is connected between a pair of conductors 53 and 54 extendingfrom the power supply 43. The connection point of the resistor 44 andthe capacitor 45 is connected to the gate electrode of an FET 46 havinga source-to-drain path connected in series with a relay 36 and a loadresistor 47 between the conductors 53 and 54. A normally open switch36-1 which is actuated by the relay 36 is connected in parallel with theseries connection of the F ET 46 and the load resistor 47 to form aself-holding contact of the relay 36. A normally open switch 16-3, arelay 38 and a normally closed switch 37-] are connected in seriesbetween the conductors 53 and 54, and a normally open switch 38-1 whichis actuated by the relay 38 is connected in parallel with the switch16-3 to form a self-holding contact of the relay 38. The switches 16-2and 16-3 are actuated by the relay 16 which also appears in FIGS. 4, 5and 6. The switch 37-1 is actuated by a relay 37 which will bedescribed. A normally open switch 38-2 which is actuated by the relay38, a resistor 48, and a capacitor 49 which also form a time constantcircuit are connected in series between the conductors 53 and 54. Thetime constants relating the relays 36 and 37 must be selected in. orderto meet the operational requirements of the device as described below.Both time constants-may be made equal by using timeconstant circuitelements having the same values. Y

The above described portion of the circuit of FIG. 7 will now beexplained with reference to the waveform diagrams of FIGS. 8-1 ,8-2 and8-3 and wherein the same time constant is used for both relays 36 and37.

When smoke comes inthe open ionization chamber 20 of an ionization smokedetector of the fire alarm system of this invention, the relay 16 in thecentral unit 2 is periodically actuated and the mode of actuation isschematically represented by the waveform (16) of FIG. 8-1, as describedabove. Such periodic actuation of the relay 16 and the similar operationof switch 16-2 results in gradual charging-up of the capacitor 45 andconsequent rise of the gate voltage and increase of the drain current ofthe FET 46. When the drain current reaches a specific value, the relay36 is energized and actuates its self-holding contact 36-1. The mode ofactuation of relay 16 is represented schematically by the waveform (36)of FIG. 8-1. The relay 16 also actuates the switch 16-3 which energizesthe relay 38. The relay 38 actuates its self-holding contact 38-1 and,at the same time, actuates the switch 38-2 to form a charging path forcapacitor 49. When the drain current of the FET 50 reaches a specificvalue by reason of the change in capacitor 49, the relay 37 is energizedand actuates the normally closed switch 37-1 to deenergize the relay 38and open the switches 38-1 and 38-2. Then, the capacitor 49 is graduallydischarged and, when the drain current of the FET 50 falls below aspecific value, the relay 37 is deenergized. Since the switch 38-2 iscontinuously actuated by the self-holding relay 38 and the charging timeconstants of both capacitors 45 and 49 are the same, the capacitor 49 ischarged more quickly than the capacitor 45 and the relay 37 is thereforeenergized earlier than the relay 36. Thus, the modes of actuation of therelay 37 and 38 are represented schematically by the waveforms (37) and(38) of FIG. 8-1, respectively. The time difierence appearing betweenthe leading edges of the waveforms (l6) and (38) corresponds to the timelag of actuation of the relay 38 after actuation of the relay 16.

However, when the ionization smoke detector is actuated erroneously dueto a damaged FET or broken insulation, the relay 16 is energizedcontinuously as shown by the waveform (16) of FIG. 8-2 and the chargingconditions of the capacitors 45 and 49 are the same except for the timelag in the charging of the capacitor 49. Therefore, the actuation of therelay 36 is earlier than the relay 37 by a time corresponding to thattime lag. Accordingly, the modes of actuation of the various relays arerepresented by FIG. 8-2, since the actuation of both relays 37 and 38are identical to those in the case of FIG. 8-1.

When the ionization smoke detector is erroneously V actuated momentarilyby an electrical noise or flow of air, the relay 16 is also energizedmomentarily but does not energize the relay 36. The modes of actuationof these relays are shown by FIG. 8-3.

With reference to the timing relation of the actuation of the relays 36and 37, it is found that the true and erroneous actuations of theionization smoke detector can be distinguished by order in which eachrelay is actuated. An example of such a discriminating circuit is shownin the lower portion of FIG. 7.

Referring again to FIG. 7, four relays 39, 40, 41 and 42 are disposedbetween the conductors 53 and 54. The relay 39 is actuated in responseto an erroneous operation informing device (not shown) and the relay 41isactuated by a fire alarm device (not shown). The relay 39 is connectedbetween conductors 53 and 54 energized.

through a normally open switch 40-1 which is actuated by the relay 40,the normally open contacts of a singlepole doublel-throw switch 37-3which is actuated by the relay 37 and a normally open switch 364 whichis actuated by the relay 36. The relay 40 is connected betweenconductors 53 and 54 through a normally closed contact of the switch37-3 and the switch 36-2. A normally open switch 40-2 which is actuatedby the relay 40 is connected across the normally closed contact of theswitch '37-3 to form a self-holding contact of the relay 40. The relay41 is connected between conductors 53 and 54 through a normally openswitch 42-1 which is actuated by the relay element 42, the normally opencontact of the single-pole double-throw switch 36-3 which is actuated bythe relay 36 and a normally open contact 37-2 which is actuated bytherelay 37. The relay 42 is also connected between conductors 53 and 54through the normally closed contacts of the switch 36-3 and the switch37-2. A normally open switch 42-2 which is actuated by the relay 42 isconnected across the normally closed contacts of the switch 36-3 to forma self-holding contact of the relay 42.

When the relay 37 is firstenergized and the relay 36 is then energizedas shown in FIG. 8-1, the switch 37-2 is closed to energize the relay 42and the relay 42 closes the switches 42-and 42-2 and is self-held by theswitch 42-2, and then the movable arm of the switch 36-3 is moved to thefixed contact connected to the relay 41 to energize the relay 41 andthereby gives a fire alarm from the fire alarm device (not shown). Astherelay 40, however, since the movable arm of the switch 37-3 is movedtothe fixed contact connected to the relay 39 before the switch 36-2 isclosed by the relay'36, the relay 40 cannot be energized at all and theswitch 40-] is therefore not closed. Therefore, the relay 39 is notFurthermore, when the relay 36 is energized and thereafter the relay 37is energized as shown in FlG. 8-2, completely opposite operation resultswith the relays 39 and 40 and the relay 39 is energized to give anerroneous operation alarm from the erroneous operation informing device(not shown) but the relay 41 is not energized. In the case of erroneousoperation as shown in FIG. 8-3, neither relay 39 nor relay 41 isenergized and no alarm is given in accordance with this circuit.

As described in the above, according to the system of this invention,true and erroneous actuations of the ionization smoke detector can bereadily distinguished from each other and a false alarm cannot beproduced by erroneous actuations of the smoke detector.

It should be noted that the circuit configurations described'above arepresented to aid in the explanation of this invention and variousmodifications and changes are possible for practicing the principle ofthis invention without departing from the scope of the invention. Forexample, the discriminator may include a conventional pulse counter forcounting the number of pulses produced by the ionization smoke detectoraccording to this invention.

What is claimed is: e

1. A fire alarm system comprising an ionization tector, an alarm devicefor producing an alarm in response to said detection output, voltagechanging meairsfor reducing said higher applied voltage below saidminimum enabling voltage to disable said detector in response to saiddetection output and restoring said higher applied voltage in responseto said disabling of said detector, and discriminating. means producinga periodic output produced by cooperation of said detector and saidvoltage changing means and a continuous output produced by erroneousoperation, said outputs selectively operating said alarm device.

2. A fire alarm system according to claim 1 wherein said alarm deviceincludes a fire alarm unit and an erroneous operation indicating unit,and said both units smoke detector having a specific minimum enablingvoltage and detecting smoke to produce a detection are drivenselectively by said discriminating means.

3. A fire alarm system according to claim 1 wherein said voltagechanging means includes a bipolar transistor controlled by saiddetection output and a Zener diode having a Zener voltage lower thansaid minimum enabling voltage, and said transistor and said Zener diodeare connected in series between the supply terminals of said appliedvoltage.

4. A fire alarm system according to claim 1' wherein said appliedvoltage changing means includes a bipolar transistor connected betweenthe supply terminals of said applied voltage and controlled by saiddetection output, a second voltage source having a voltage lower thansaid minimum enabling voltage, and means for interchanging said appliedvoltage source and said second voltage source in response to theconduction current of said transistor.

5. A fire alarm system according to claim 4 wherein said interchangingmeans includes a relay and said relay produces an output for drivingsaid discriminating means. i

6. A fire alarm system according to claim 1 wherein said applied voltagechanging means includes a bipolar transistor connected between thesupply terminals of said applied voltage and controlled by said,detection output, and a resistance element connected in series to saidsupplied voltage source, the value of said resistance element beingselected to make the applied voltage of said detector during productionof the detection output lower than said minimum enabling voltage.

7. A fire alarm system according to claim 6 wherein said resistanceelement consists of the electromagnetic winding of a relay for producingan output for driving said discriminating means.

. 8. A fire alarm system according to claim 1 wherein saiddiscriminating means includes a first capacitor which starts chargingconcurrently with initiation of said periodic output and continuousoutput and continues charging during the duration of said outputs, afirst relay energized in response to a specific charge in said firstcapacitor, a second capacitor which starts charging with a specificdelay from the initiation of said outputs and continues chargingconstantly, a second relay energized in response to a specific charge insaid second capacitor, and means for discriminating the order of saidenergization of said first and second relays and driving said alarmdevice selectively, the time conto the connection point of said bothionization chambers and a source-to-drain path connected between saidterminals and having a specific minimum enabling voltage, and a bipolartransistor and a Zener diode connected in series between said terminals,said bipolar transistor being controlled by said field effect transistorand said Zener diode having a Zener voltage lower than said minimumenabling voltage.

I II! IF

1. A fire alarm system comprising an ionization smoke detector having aspecific minimum enabling voltage and detecting smoke to produce adetection output under the condition that the applied voltage is higherthan said minimum enabling voltage, a voltage source for supplying avoltage at least higher than said minimum enabling voltage to saidionization smoke detector, an alarm device for producing an alarm inresponse to said detection output, voltage changing means for reducingsaid higher applied voltage below said minimum enabling voltage todisable said detector in response to said detection output and restoringsaid higher applied voltage in response to said disabling of saiddetector, and discriminating means producing a periodic output producedby cooperation of said detector and said voltage changing means and acontinuous output produced by erroneous operation, said outputsselectively operating said alarm device.
 2. A fire alarm systemaccording to claim 1 wherein said alarm device includes a fire alarmunit and an erroneous operation indicating unit, and said both units aredriven selectively by said discriminating means.
 3. A fire alarm systemaccording to claim 1 wherein said voltage changing means includes abipolar transistor controlled by said detection output and a Zener diodehaving a Zener voltage lower than said minimum enabling voltage, andsaid transistor and said Zener diode are connected in series between thesupply terminals of said applied voltage.
 4. A fire alarm systemaccording to claim 1 wherein said applied voltage changing meansincludes a bipolar transistor connected between the supply terminals ofsaid applied voltage and controlled by said detection output, a secondvoltage source having a voltage lower than said minimum enablingvoltage, and means for interchanging said applied voltage source andsaid second voltage source in response to the conduction current of saidtransistor.
 5. A fire alarm system according to claim 4 wherein saidinterchanging means includes a relay and said relay produces an outputfor driving said discriminating means.
 6. A fire alarm system accordingto claim 1 wherein said applied voltage changing means includes abipolar transistor connected between the supply terminals of saidapplied voltage and controlled by said detection output, and aresistance element connected in series to said supplied voltage source,the value of said resistance element being selected to make the appliedvoltage of said detector during production of the detection output lowerthan said minimum enabling voltage.
 7. A fire alarm system according toclaim 6 wherein said resistance element consists of the electromagneticwinding of a relay for producing an output for driving saiddiscriminating means.
 8. A fire alarm system according to claim 1wherein said discriminating means includes a first capacitor whichstarts charging concurrently with initiation of said periodic output andcontinuous output and continues charging during the duration of saidoutputs, a first relay energized in response to a specific charge insaid first capacitor, a second capacitor which starts charging with aspecific delay from the initiation of said outputs and continuescharging constantly, a second relay energized in response to a specificcharge in said second capacitor, and means for discriminating the orderof said energization of said first and second relays and driving saidalarm device selectively, the time constants of charging said first andsecond capacitors being selected so that said second relay is energizedearlier in the case of charging according to said periodic output andsaid first relay is energized earlier in the case of charging accordingto said continuous output.
 9. An ionization smoke detector used in thesystem according to claim 1, including a pair of terminals, a closedionization chamber and an open ionization chamber connected in seriesbetween said terminals, a field effect transistor having a gateelectrode connected to the connection point of said both ionizationchambers and a source-to-drain path connected between said terminals andhaving a specific minimum enabling voltage, and a bipolar transistor anda Zener diode connected in series between said terminals, said bipolartransistor being controlled by said field effect transistor and saidZener diode having a Zener voltage lower than said minimum enablingvoltage.